tests: add reference_field_must_be_initialized.vv

vlib/v/checker/tests/reference_field_must_be_initialized.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/reference_field_must_be_initialized.v:8:7: error: reference field `Node.next` must be initialized
+    6|
+    7| fn main(){
+    8|     n := Node{ data: 123 }    
+                ~~~~~~~~~~~~~~~~~
+    9|     eprintln('n.data: $n.data')
+   10| }

vlib/v/checker/tests/reference_field_must_be_initialized.vv

@@ -0,0 +1,10 @@
+module main
+struct Node {
+    data int
+	next &Node = 0
+}
+
+fn main(){
+	n := Node{ data: 123 }    
+    eprintln('n.data: $n.data')
+}